1. Field of the Invention
The present invention relates to an apparatus and method for sorting particles, and more particularly, concerns an apparatus and method for sorting particles moving in a liquid stream, according to differences in particle parameters, and collecting same in a fashion to neutralize the collection well.
2. Description of the Prior Art
Flow analysis of particles has been employed in the determination of characteristics of individual particles. Such analysis is most useful in analyzing characteristics of cells for the collection of information which would be useful in areas of research, hemotology, immunology and the like. The researcher may be interested, for example, in determining specific characteristics of individual cells so that the cells may be classified, identified, quantified and then sorted for further investigations, analyses or other activities such as the production of monoclonal antibodies. There are a number of well-known cell sorters available to the researcher at present utilizing flow cytometry techniques for the analysis of characteristics of individual cells and the subsequent sorting of those cells of particular interest. One such cell sorter is known as the FACS fluorescence-activated cell sorter, sold by Becton Dickinson FACS Systems, Sunnyvale, Calif.
The FACS sorter, and others operating under similar principles, employs elements which establish an electrostatic field as the basis for particle sorting. In this type of particle sorter, particles of selective interest are charged with positive or negative charges; charged and uncharged particles are separated by passing them through an electrostatic field established between two oppositely charged plates. Once in this field, particles of a given polarity are deflected toward the oppositely charged deflection plate and can thus be separately collected into corresponding collection wells.
However, some problems have been encountered in this type of particle sorter. One specific problem relates to the collection of particles, all of which have the same electrical charge, in a collection well which is the typical and standard manner of collecting particles which have been separated based on electrical charge. As particles charged with the same electrical polarity accumulate in respective collection wells, the net result is that the collection well itself assumes the electrical polarity of the particles being collected therein. Once this electrical charge of the collection well is established, incoming particles of the same polarity are deflected away since there is an electrostatic force operating to separate charges of like nature. It can be appreciated that an electrically charged collection well can cause the loss of particles which normally would be collected in the collection well.
Particle sorters of the type relying upon electrostatic separation of particles are described in U.S. Pat. Nos. 3,380,584; 3,710,933; 3,826,364; 4,148,718; 4,230,558; and 4,318,480. The particle sorters described in these patents, inasmuch as they rely upon an electrostatic field for separating and sorting particles, are susceptible to the above-described problem wherein the collection wells become charged as particles of a specific polarity are accumulated.
There have been some attempts to neutralize or discharge collection wells associated with electrostatic particle sorters. For instance, it has been known to immerse an inert conductor, such as a platinum wire or foil, into the collection well, and then connect this conductor to ground potential. This technique has two significant disadvantages: first, the configuration of the grounding connector has to be made specific for each collection medium used, and second, the requirement for sterility or cleanliness of collection becomes increasingly difficult to satisfy. Another approach to neutralize or discharge collection wells has been the use of intrinsically conductive collection vials such as glass or plastic doped with a conductive material, and then connecting the vial holder to ground potential. Even though these approaches to neutralize or discharge collection wells have, to some extent, been feasible, a more effective technique is still required without the disadvantages of the presently known techniques of neutralization of collection wells. It is to such an improved technique for neutralizing collection wells that the present invention is directed.